In order to implement a parking brake functionality in motor vehicles, two actuators of the service brake are usually secured by means of a suitable device. In order to arrest the vehicle, a brake application force is applied via actuators of the service brake, and the service brake is subsequently locked mechanically. The provision of the brake application force requires power which has to be made available by the on-board power system. The largest amount of power is not required here for holding the brake application force which is to be set but rather for accelerating the masses and moving the brake components. If two electromechanical actuators are driven simultaneously, the electrical power which is to be applied is multiplied.
If the vehicle then only has a an on-board electrical power system with compromised functionality (e.g. weak battery) and a request for the activation of the parking brake function occurs, simultaneous activation of both electromechanical actuators can lead to a failure of the on-board power system. In the text which follows it will be assumed that the vehicle has, by way of example, two electromechanical actuators which implement the parking brake function.
In order to reduce the electrical power which has to be applied it is possible that the two actuators which implement the parking brake function are respectively actuated and locked in succession. This means that firstly the first actuator is actuated and locked, and the second actuator is then actuated and locked.
This approach can lead to the following disadvantageous situation in the case of a low power on-board power system or one which has been compromised in its functioning. The motor vehicle may be located, for example, on an incline which is so large that the mass of the vehicle cannot be held by one parking brake alone. In order to save the on-board power system, a first parking brake is initially locked. Before the second parking brake can be locked, the on-board power system could fail, with the result that the power which is necessary to release the already locked parking brake is also no longer available.
In this situation, the vehicle cannot be held on the slope by the one locked parking brake. At the same time, the driver can no longer move the vehicle while just one parking brake is locked. In the worst case, the car therefore begins to slip in an uncontrolled fashion. Such a situation can cause serious injury to persons and material damage. Furthermore, in such a state the vehicle cannot be towed away by another motor vehicle, for example using a towing cable or a towing bar.
The present invention is therefore based on the object of making available a method for operating a braking system which reliably prevents the occurrence of a state of a motor vehicle in which the motor vehicle is neither securely arrested nor maneuverable. Furthermore, the invention is to specify an associated braking system in which such a method is to be carried out, as well as a motor vehicle having such a braking system.
With respect to the method of this invention, the above-mentioned object is achieved in that when a parking brake function is carried out, a first method step is carried out in which the at least two actuators are actuated in succession, and subsequently a second method step is carried out in which it is checked whether the at least two actuators have achieved a respectively predefined brake application force, and in the event of the at least two actuators having achieved a respectively predefined brake application force, the at least two actuators are locked in a third method step.
The invention is based on the idea that an indispensable safety requirement of modern motor vehicles is that they can be arrested or parked in a reliable way. In particular, on steep slopes or on a precipitous underlying surface it is necessary for the parking brake or parking brake function which is provided for this purpose to secure the motor vehicle against slipping away. At the same time, a malfunction of the parking brake must not lead to a situation in which the vehicle becomes (to a limited degree) incapable of being maneuvered or controlled.
In modern motor vehicles, increasing use is being made of electromechanical brakes (brakes which can be activated electromechanically), in which an electrically driven actuator (for example in the form of an electric motor) applies the brake. The electrical power which is required for the application of the brake is drawn from the on-board electrical power system.
In such braking systems there is then the risk that in the case of a weak on-board power system, the on-board power system will fail before all the brakes can be satisfactorily applied and locked. This can have the result, as mentioned above, that a motor vehicle is inadequately secured against slipping away, and when a slipping process occurs, persons in the surrounding area are put at risk.
As has now been recognized, the above-described state can be avoided by virtue of the fact that only a minimum possible power is required in the on-board power system at any time during the arresting process if the actuators are successively actuated. Therefore, only power for a single brake application process is required at one time. Furthermore, by checking the brake application force before the locking step, it is possible to avoid individual actuators being locked even though they have not reached the required brake application force. In the event of a possible failure of the on-board power system into such a situation, the actuators can then no longer be released and the vehicle is incapable of being maneuvered and at the same time is not securely arrested.
In the event that not all the actuators have achieved a predefined brake application force, all the actuators are advantageously released or opened in a third step. As a result, it is possible to avoid the motor vehicle starting to rotate and/or slip since, for example, if only one brake is satisfactorily applied and cannot alone hold the motor vehicle. As a result of the release of the brakes, it is possible to ensure that the driver can move the motor vehicle and can arrest it securely at another position, for example by engaging a gearshift.
When all the actuators have reached the predefined brake application force, in the third step locking devices are preferably activated essentially simultaneously, as result of which the motor vehicle is as quickly as possible in a locked and secured state.
In order to prevent undesired sudden and hazardous braking of a moving motor vehicle, the parking brake function is advantageously carried out only when the motor vehicle is essentially in the stationary state. The stationary state of the motor vehicle may be detected, for example on the basis of the signals of wheel-speed sensors which are mounted on the wheels of the motor vehicle.
The predefined brake application force is advantageously selected to be essentially the same for all the actuators and here is preferably in a range from 5 kN to 30 kN, in particular in certain applications from 10 kN to 25 kN.
In one preferred embodiment of the method for a motor vehicle having an on-board power system in accordance with this invention, the on-board power system of the motor vehicle is checked while the parking brake application function is being carried out.
Checking of the on-board power system proves advantageous since electrical power, which has to be made available by the on-board power system, is necessary for producing the brake application force of the actuators. The greatest power is not required here for maintaining the brake application force which is to be set (the predefined brake application force), but rather for accelerating the masses and setting brake components into motion (for example the components of an electric motor). Checking the on-board power system therefore already provides conclusive information, before the actual brake application processes, as to whether under certain circumstances it is possible to obtain enough power to simultaneously satisfactorily actuate and lock the actuators.
If the checking of the on-board power system has revealed that its function is compromised, the respective method steps illustrated above are advantageously executed and otherwise, that is to say if the on-board system is not compromised, replaced by a single method step in which all the actuators are essentially simultaneously actuated and locked in a normal application process. In the case of a fully functionally capable on-board power system, it is possible in this way to easily and quickly park or arrest the motor vehicle. If, on the other hand, the on-board power system's function is compromised, the steps of the method are executed according to which the actuators are successively actuated, their brake application force is checked and then, if appropriate, the actuators are locked.
In the case of sequential actuation of the actuators, in each case only the power for a single actuator is required from the on-board power system. The simultaneous actuation of, for example, two actuators would, in a first approximation, require twice the power compared with operating one. The sequential actuation of the actuators can therefore also be successful if the on-board power system can make available just a proportion of its power which is available in the normal state. If the on-board power system fails before all the actuators have reached the required brake application force, the situation is therefore prevented in which the vehicle is only incompletely arrested and, for example, starts to slip on a slope, as a result of only some of the actuators being locked. In the case of an on-board power system which has failed, it is possible, on the other hand, that the actuators which have already been locked can no longer be released, with the result that the vehicle cannot be brought under control.
With respect to the braking system, the above-mentioned objective is achieved according to the invention in that said braking system comprises a control device, connected on the signal side to the wheel brakes, for actuating the actuators and the locking devices.
The control device is advantageously embodied as a brake control device and actuates the electromechanical actuators in order to execute a service brake function.
With respect to the motor vehicle with an on-board power system, the above-mentioned objective is achieved according to the invention by a braking system as represented above.
In one preferred embodiment, the control device comprises an on-board power system checking module which is electrically connected to the on-board power system of the motor vehicle. In combination or as an alternative thereto, the control device is connected on the signal input side to an on-board power system checking device which is electrically connected to the on-board power system.
In one advantageous embodiment, the motor vehicle comprises a front axle and a rear axle and two electromechanical wheel brakes which are assigned to the rear axle and each comprise a locking device for implementing the parking brake functionality, and two hydraulic brakes which are assigned to the front axle.